Online inspection of printed materials is realized in the prior art through the use of either a densitometer attached to the printing press that reads small area of ink along the edge of the substrate, known as test targets or through the use of an electronic color video or color digital camera that reads either the test targets or specified areas within the printed image. Disclosures of such prior art are found in U.S. Pat. Nos. 4,289,405; 5,163,012; and 5,774,225.
In those methods that utilize a color video camera, the camera is used as a light sensor with three wide-band light detectors, commonly referred to as Red, Green or Blue (RGB) with spectral sensitivities that peak in the “blue”, “green” or “red” regions of the visible spectrum. The light sensor integrates or sums all of the light rays with wavelengths within its passband. The camera sensors are then used to approximate the responses of a Standard ISO Status Density, as defined in ISO 5/3. It is important to note that the spectral response of the three camera sensors only approximate the ISO Status Density spectral curves.
The densitometer or the camera measures “substrate relative” density. That is, the camera is first pointed to the unprinted substrate and the light projected onto the substrate. The projected light that is reflected from the substrate is collected by camera in each of its three sensors. Typical RGB camera signals are binary coded values with a range of 0 to 255 (8 bits). The camera is adjusted so that a perfect white object will read RGB values (255, 255, 255). The values are normalized so that the perfect white will have relative values of (1.0, 1.0, 1.0) as is disclosed in U.S. Pat. Nos. 5,724,259 and 5,767,980. The normalized values of the sensors are converted into density by computing the negative of the logarithm of the sensor value. Next, a printed area is move into the field of view of the camera and the light projected onto that area. The camera captures the light reflected from the printed area, comprised of the ink and the substrate. The camera readings are again converted to density. The previously computed substrate density is then subtracted from the ink-on-substrate density to leave only the density of the ink. The density of the ink is assumed to be proportional to the thickness of the ink layer.
Because of the differences between the camera sensors and an ISO Status Densitometer, it is not possible to simultaneously obtain colorant concentration and ink film thickness. On a commercial offset press the only parameter that is available to the pressman to control is the weight of ink applied to the substrate which modulates the ink film thickness. Accordingly, there is a need in the printing industry to have a press inspection system that measures and tacks the color and the concentration of the inks as they are being printed.